Local area networks (LANs) to enable local communication among computing stations and computer related devices are well known in the art. The reference entitled "An Introduction to Local Area Networks" by D. Clark et al., Proceedings of the IEEE, Vol. 66, No. 11, pps. 1497-1517, November 1978, provides a general description of local area networks and their operation and is incorporated in its entirety herein by reference. LANs are implemented using a variety of network topologies and control structures, the control structure determining how the different devices connected to a network achieve access thereto and communicate with one another. Such topologies and control structures are described in the above-cited reference by Clark et al.
One control structure that finds wide use is the carrier-sense multiple-access with collision detection (CSMA/CD) protocol. Examples of this are found in LANs following IEEE 802.3 and the Ethernet (a trademark of Xerox Corporation) specifications for a CSMA/CD network. The CSMA/CD protocol defines LAN operation in the data link layer of the International Standards Organization (ISO) seven protocol layer open system interconnection model. A LAN such as an Ethernet LAN defines the physical (i.e., transmission medium) and data link layers of the ISO seven layer model. One benefit of adopting a standardized LAN for the two lowest protocol layers is that it enables a number of subnetworks to share the same LAN. Such subnetworks can then communicate with each other via the LAN provided they have compatible client protocol layers, i.e., the protocol layers above the data link layer in the ISO model. In the case of a large user, e.g., a large corporation, it often occurs that a number of such LANs are installed, each LAN being primarily intended for interconnecting specific subnetworks of devices having compatible client protocols. Subsequently, a need develops to establish communication between the discrete LANs.
To meet this need and for a variety of other reasons described in the above-cited Clark et al. reference, it may be desirable to interconnect discrete LANs by means of communication bridges. A bridge interconnecting two LANs acts as a filter by transmitting only message frames having a destination address of a device located on the far side of the bridge. A bridge typically includes two transceivers, each for transmitting message frames to and receiving frames from one of the two LANs the bridge interconnects. Note that while the transceivers are essential components that enable receipt and transmission of message frames, they are not always considered an integral part of a bridge by those skilled in the art. Bridges also include means for interfacing with the respective control structures of the LAN they interconnect as well as means for performing the message frame filtering discussed above.
Thus, the situation is created where bridges are used to interconnect discrete LANs each having devices connected thereto with client protocol layers compatible with other devices on the same LAN but not necessarily compatible with devices on other LANs. Such bridges are typically adapted to be specific to the client layer protocols being used on the LANs. This may be accomplished by applying a separate bridge for each protocol involved or a complex bridge that can handle all of the protocols. A bridge that is independent of the client layer protocols is desirable since such a bridge would be simpler and less expensive to construct and could be applied between LANs independently of the client layer protocols specific thereto.
In large communication systems consisting of multiple LANs and bridges, the total number of communicating devices can be substantial with many of these devices communicating with many other devices on the system irrespective of the device locations. As a result, each bridge interconnecting its two proximate LANs also receives message frames from devices connected to LANs other than the proximate LANs, the frames being intended for yet other LANs. Thus, it is a desirable feature of such a bridge to have a capacity to handle a large number of message frames in as short a time period as possible so that the bridge does not become a communication bottleneck, i.e., a large bandwidth is desirable.
Client layer protocols sometimes require a receiving device to acknowledge to the sending device that a message frame has been received. Typically, a relatively short time period is allocated by the sending device within which to receive the acknowledgement. Where acknowledgement is not received within this time period, the sending device assumes the message frame transmission failed and a retransmission may be attempted. In consideration of such client layer protocols, bridge operation must be fast enough so as to not introduce a delay resulting in an apparent transmission failure.